Commercially available bags for containing foodstuffs are usually left open at their one ends. Accordingly, since external atmosphere invades to contaminate the inside of bags before they are used for preservation of foodstuffs, they are not suitable to the long time storage for foodstuffs. The present applicant has already filed patent applications concerning the subject for overcoming the foregoing drawbacks (refer to U.S. patent application Ser. No. 561,975 and Ser. No. 677,577 as a continuation-in-part application thereof). Further, the known bags are not yet quite satisfactory, because when the bag is stored while being laid laterally in a refrigerator or the like or carried about after it has been filled with foodstuffs through the upper opening and folded back the opening and then sealed at the folded-back portion with an adhesive tape or the like, the foodstuffs contained will bleed to leak through the folded portion at both longitudinal edges of the bag even when a tight sealing is applied there. Furthermore, the bag in the prior art has a drawback in that the operation for sealing the entrance of the bag after filled the flowable foodstuff is complicate and troublesome.